A number of cannula materials are known which will soften significantly on being inserted into a patient due to contact with an aqueous liquid (hydrophilic cannulae) or on being heated to body temperature (shape-memory material). Such cannulae will also usually swell (increase wall thickness and inner and outer diameters) when this occurs. If such cannulae are of a hydrophilic material the swelling is due to uptake of water; if of shape-memory material due to return to a previous thickness and previous diameters on being heated to a glass transition temperature. The use of cannulas which will soften and swell on being contacted with an aqueous liquid, for example on the distal end portion of the cannula being inserted in a blood vessel, has a number of advantages. For example, a relatively small puncture can be made into a blood vessel whereupon, after removal of the needle, the cannula will swell to provide a relatively large internal diameter duct as compared to the size of the needle used. Also, the softening is useful in that trauma is thereby kept relatively low.
Unfortunately, a problem exists with softenable cannulae of the nature just described. This problem is that once the cannulae have softened with a softening ratio of at least 2:1 they become very easily distorted or kinked over the entire distance from where they exit the body to the hub to which their proximal ends are attached.
With non-softening cannulae problems of kinking also exist, but only to a significant extent at the point where the cannula proximal end is attached to the hub. Such problems have been incidentally solved while providing cannulae of desired lubricity (see U.S. Pat. No. 4,381,008, issued Apr. 26, 1983 to J. J. Thomas and N. Sobel) by cannulae which are thicker and of greater diameter just where they attach to the hub. The thickened and larger diameter portion of the cannula does not extend far from the hub, and particularly does not extend into the body, since the only problem encountered relates to kinking at the connection to the hub. With such prior art cannulae the problem of distortion or kinking along the entire length of exposed cannula is either not encountered or is of such little significance as to not be considered a problem at all. This is because these catheters are made from a relatively stiff material having a softening ratio of less than 2:1 whereby the only area of concern is the portion of the cannula as it exits the hub.
The present invention is directed to overcoming one or more of the problems set forth above.